Particle beams are often used in the study of nuclear physics and they are difficult to analyze because their intensity levels are low compared to environmental noise. Because of their low intensity levels, it is important to minimize or compensate for the noise generated by scanners that are used in determining the profiles of these beams.
Ion beam profile scanners, such as the one disclosed in U.S. Pat. No. 3,789,298, issued Jan. 29, 1974 to Herb, experience system noise from sources such as the motion of the probe through the beam, changing capacitance between the moving probe and the charged detector, and pickup from line power. Many of these noise sources have frequencies at or near the range of typical beam sample rates (scan rates) of 10-30 Hz and the noise frequency will often drift. For this reason, system noise is often difficult to filter out from the desired signal.
System noise may be reduced within the ion beam profile scanner. In prior art devices, such as the Herb patent referenced above, the detector is generally a cylinder coaxial with the ion beam and perpendicular to the axis of rotation of the probe. Using this detector construction, it was discovered that systematic noise may be reduced by using an uncharged cylindrical detector. But, since charged particles are to be detected, using an uncharged detector places a severe limitation on the sensitivity of the detector. The loss in sensitivity, however, is justified by the noise reduction of this design.
In one ion beam detector a charged plate detector is configured and oriented to present a substantially symmetric view to a rotating ion beam probe. One such detector is a circular metal disc. As the probe rotates, there is very little or no changing capacitance between the detector and the probe. Since the probe may not rotate with perfect symmetry in a perfect plane, however, the detector may have to be mechanically adjusted until system noise is reduced to acceptable levels. Even with the reduced noise levels achieved by the use of symmetric view collectors, there remains a substantial amount of noise having a frequency at or near the scan rate of the probe.